Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Horizontally constructed continuous steam generator and method for the
operation
thereof
FIELD OF THE INVENTION
The invention relates to a continuous steam generator wherein a continuous
evaporator heating surface comprising a plurality of parallel-connected steam
generator tubes providing a flow path for a flow medium is disposed in a hot
gas duct
through which hot gas can flow in an approximately horizontal direction.
BACKGROUND OF THE INVENTION
In a gas and steam turbine plant, the heat contained in the expanded working
medium or heating gas from the gas turbine is utilized for the generation of
steam for
the steam turbine. Heat transfer takes place in a waste-heat steam generator
disposed downstream of the gas turbine and in which a number of heating
surfaces
for water preheating, steam generation and steam superheating are normally
disposed. The heating surfaces are connected into the water/steam circuit of
the
steam turbine. The water/steam circuit normally contains several, e.g. three,
pressure
stages, in which case each pressure stage may have an evaporator heating
surface.
For the steam generator mounted downstream of the gas turbine on the heating-
gas
side as a waste-heat steam generator, a number of alternative design concepts
are
suitable, namely configuration as a continuous steam generator or as a
circulation
steam generator. In the case of a continuous steam generator, the heating of
steam-
generator tubes provided as evaporator tubes results in evaporation of the
flow
medium in the steam generator tubes in a single pass. In contrast, in the case
of a
natural or forced circulation steam generator,
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the circulating water is only partly evaporated as it passes
through the evaporator tubes, the water that is not evaporated
being is re-fed to the same evaporator tubes for further
evaporation after separation of the generated steam.
A continuous steam generator, in contrast to a natural or
forced circulation steam generator, is not subject to any
pressure limitation, which means that live-steam pressures
well above the critical pressure of water (Pcri 221 bar) -
where there is only a slight difference in density between a
fluid-like medium and a steam-like medium - are possible. A
high live steam pressure promotes high thermal efficiency and
therefore low C02 emissions from a fossil-fired power plant. In
addition, a continuous steam generator has a simple type of
construction compared with a circulation steam generator and
can therefore be manufactured particularly inexpensively. The
use of a steam generator designed according to the continuous
principle as the waste-heat steam generator of a gas and steam
turbine plant is therefore particularly advantageous for
achieving a high overall efficiency of the gas and steam
turbine plant using a simple type of construction.
Particular advantages in terms of manufacturing costs, but
also of maintenance required, are provided by a horizontally
constructed waste-heat steam generator in which the heating
medium or heating gas, i.e. the exhaust gas from the gas
turbine, is passed through the steam generator in an
approximately horizontal flow direction. However, with a
horizontally constructed continuous steam generator the steam
generator tubes of a heating surface may be subjected to
markedly differential heating depending on their positioning.
Particularly in the case of steam generator tubes connected to
a common header on the output side, differential heating of
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individual steam generator tubes may result in a combining of
steam flows with greatly differing steam parameters and
therefore undesirable efficiency losses, in particular
comparatively diminished effectiveness of the heating surfaces
affected and consequently reduced steam generation.
Differential heating of adjacent steam generator tubes may
also result in damage to the steam generator tubes or the
header, particularly in the region where they discharge into
headers. The per se desirable use of a horizontally
constructed continuous steam generator as a waste-heat steam
generator for a gas turbine can therefore entail considerable
problems in terms of sufficiently stabilized flow control.
EP 0 944 801 B1 discloses a steam generator suitable for a
horizontal type of construction and additionally having the
abovementioned advantages of a continuous steam generator. To
this end, the disclosed steam generator is designed in respect
of its continuous evaporator heating surface in such a way
that a steam generator tube heated more than another steam
generator tube of the same continuous evaporator heating
surface has a higher throughput of flow medium than the other
steam generator tube. If differential heating of individual
steam generator tubes occurs, the continuous evaporator
heating surface of the steam generator disclosed therefore
exhibits, in its flow characteristic typical of a natural
circulation evaporator heating surface (natural circulation
characteristic), a self-stabilizing behavior resulting in a
matching of the outlet-side temperatures even to
differentially heated steam generator tubes connected in
parallel on the flow medium side without the need for external
intervention. However, this concept requires that the
disclosed steam generator be designed for feeding with flow
medium having comparatively low mass flow density.
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SUMMARY OF THE INVENTION
The object of the invention is therefore to specify a continuous steam
generator of the
abovementioned type which ensures particularly high operational reliability
even when
fed with flow medium with comparatively high mass flow densities. In addition,
a
particularly suitable method for operating the steam generator of the
abovementioned
type shall be set forth.
To achieve this object with respect to the continuous steam generator, the
continuous
evaporator heating surface comprises a first heating surface segment through
which the
flow medium can flow countercurrently to the heating gas duct and another
heating
surface segment connected upstream of said heating surface segment on the flow
medium and heating gas side, the flow-medium-side outlet of the first heating
gas
segment viewed in the heating gas direction being positioned such that the
pressure-
dependent saturated steam temperature arising at the outlet of the continuous
evaporator heating surface during operation deviates by less than a predefined
maximum deviation of no more than 70 C from the heating gas temperature
obtaining at
the position of the outlet of the heating surface segment during operation.
The invention is based on the consideration that, if the continuous evaporator
heating
surface is fed with comparatively high mass flow densities, locally
differential heating of
individual tubes could affect the flow conditions in such a way that less flow
medium
flows through more strongly heated tubes and more flow medium flows through
less
strongly heated tubes. More strongly heated tubes would in this case be cooled
worse
than less strongly heated tubes, with the result that the temperature
differences
occurring
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would be automatically amplified. In order to be able to
effectively meet this eventuality even without actively
influencing the flow conditions, the system must be suitably
designed for fundamental and total limiting of possible
AMENDED SHEET
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temperature differences. To this end, the knowledge can be
used that, at the outlet from the continuous evaporator
heating surface, the flow medium must have at least the
saturated steam temperature essentially due to the pressure in
the steam generator tube. On the other hand, however, the flow
medium can have a temperature no higher than that of the
heating gas at the point of outlet of the flow medium from the
continuous evaporator heating surface. By suitably matching
these two temperature limits generally defining the possible
temperature interval, the maximum possible temperature
imbalances can therefore also be suitably limited. By
subdividing the continuous evaporator heating surface into an
outlet-side counterflow segment and another segment upstream
of it on the heating gas and media side, the outlet is freely
positionable in the heating gas direction, with the result
that an additional design parameter is available, a
particularly suitable means of matching the two temperature
limits being the selective positioning of the outlet of the
continuous evaporator heating surface in the flow direction of
the heating gas.
The positioning of the outlet of the continuous evaporator
heating surface in relation to the temperature profile of the
heating gas in the gas flue is advantageously selected such
that a maximum deviation of approximately 50C is maintained so
as to ensure particularly high operating reliability in
respect of available materials and further design parameters.
A particularly simple and therefore also robust type of
construction can be achieved by making the heating surface
particularly simple in respect of collecting and distributing
the flow medium. To this end, the heating surface is suitably
implemented for performing all the process steps of complete
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evaporation, i.e. pre-heating, evaporation and at least
partial superheating, in a single stage, i.e. without
interposed components for collecting and/or distributing the
flow medium. A number of steam generator tubes therefore
advantageously comprise a plurality of riser and downcomer
tube sections connected in series in an alternating manner on
the flow medium side.
In this arrangement heating takes place both in the riser and
downcomer tube sections. However, such a connection of steam
generator tubes in which heating of downflow tube sections
also takes place generally involves the risk of flow
instabilities occurring. It has been found that the occurrence
of steam bubbles in downflow steam generator tubes may be
regarded as one of the possible causes of this. If steam
bubbles were to form in a downflow steam generator tube, they
could rise in the water column present in the steam generator
tube, thereby performing a movement counter to the flow
direction of the flow medium. In order to consistently prevent
any such movement of steam bubbles possibly present against
the flow direction of the flow medium, forced entrainment of
the steam bubbles in the actual flow direction of the flow
medium must be ensured by suitable specification of operating
parameters. This can be achieved by arranging that the
continuous evaporator heating surface is fed in such a way
that the flow rate of the flow medium in the steam generator
tubes has the desired entrainment effect on any steam bubbles
present. A comparatively high flow rate even in the first
downflow steam generator tube can be achieved in a very simple
manner by means of comparatively strong heating of the steam
generator tubes at the flow-medium-side inlet and the
resultant rapid increase in the steam content of the flow
medium. For this purpose the flow-medium-side inlet of the
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continuous evaporator heating surface is advantageously
implemented as a riser tube section and disposed close to the
heating-gas-side inlet of the continuous evaporator heating
surface in such a way that, during operation, the flow medium
flowing through the steam generator tubes has a flow rate
higher than a predefined minimum rate at the inlet of the
first downcomer tube section.
The first riser and downcomer tube sections constitute an
additional heating surface segment disposed in a cocurrent
flow configuration, hereinafter also referred to as a
cocurrent segment, which advantageously precedes, on the flow
medium side, the heating surface segment advantageously
disposed in a countercurrent flow configuration, hereinafter
also referred to as a countercurrent segment. By means of such
an arrangement of the segments in the heating gas duct, the
advantage of a pure countercurrent flow configuration, that of
effectively transferring the heat of the exhaust gas to the
flow medium, is largely retained while at the same time
achieving a high inherent safeguard against damaging
temperature differences at the flow-medium-side outlet.
In an alternative advantageous embodiment, however, the
additional heating surface segment can also be connected
countercurrently with respect to the heating gas direction.
The steam generator is usefully employed as a waste-heat steam
generator of a gas and steam turbine system, the steam
generator being advantageously connected downstream of a gas
turbine on the heating gas side. In this arrangement, it is
advisable for supplementary firing for increasing the heating
gas temperature to be disposed downstream of the gas turbine.
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In respect of the method, the abovementioned object is
achieved by educting the flow medium from the continuous
evaporator heating surface in the heating gas direction at a
position at which the heating gas temperature obtaining during
operation deviates by less than a predefined maximum deviation
of no more than 70 C from the saturated steam temperature
arising during operation as a result of the pressure loss in
the continuous evaporator heating surface.
Upstream of its outlet from the continuous evaporator heating
surface, the flow medium is advantageous fed countercurrently
to the heating gas, a maximum deviation of approximately 50 C
being specified in an additional or alternative advantageous
embodiment.
In order to consistently prevent any flow instabilities from
occurring, the flow medium is advantageously exposed to strong
heating at or immediately after the inlet to the continuous
evaporator heating surface in such a way that it exhibits a
flow rate of more than a specified minimum rate in a first
riser tube section of the relevant steam generator tube.
Advantageously the flow rate required for the entrainment of
steam bubbles produced in the relevant first downcomer tube
section is predefined. The continuous evaporator heating
surface is therefore fed in such a way that, even in the first
downflow steam generator tube, the comparatively high flow
rate has the desired entrainment effect on any steam bubbles
present, thereby reliably preventing flow instabilities caused
by any movement of rising steam bubbles against the flow
direction of the flow medium.
AMENDED SHEET
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The advantages achieved with the invention are in particular that, by means of
the
now provided positioning of the flow-medium-side outlet of the continuous
evaporator
heating surface, adapted to the temperature profile of the heating gas in the
gas flue,
the overall achievable temperature interval between saturated steam
temperature of
the flow medium and heating gas temperature at the point of outlet during
evaporation of the flow medium is comparatively tightly limited, so that only
small
outlet-side temperature differences are possible irrespective of the flow
conditions,
thereby ensuring adequate matching of the temperatures of the flow medium in
every
operating state. However, it is also ensured, moreover, that the possible
outlet
temperatures are limited in absolute terms, so that they remain reliably
within the
permissible temperature limits predefined by the material properties.
According to one aspect of the present invention, there is provided a
continuous
steam generator, comprising: a heating gas duct through which heating gas
flows in
an approximately horizontal direction; a continuous evaporator heating surface
disposed in the heating gas duct and comprising a number of parallel-connected
steam generator tubes that provide a flow path for a flow medium; a first
heating
surface segment incorporated with the continuous evaporator heating surface
and
through which the flow medium can flow countercurrently to the heating gas
duct; and
a second heating surface segment arranged upstream of the first heating
surface
segment in the heating gas duct; wherein a flow medium side inlet of the
continuous
evaporator heating surface is disposed close to a heating gas side inlet of
the
continuous evaporator heating surface, and wherein the flow medium side inlet
of the
continuous evaporator heating surface comprises a riser tube section.
According to another aspect of the present invention, there is provided a
method for
operating a continuous steam generator, comprising: providing a heating gas
duct
through which heating gas flows in an approximately horizontal direction and
which
has a continuous evaporator heating surface comprising a number of parallel-
connected steam generator tubes which provide a flow path for a flow medium;
and
educting the flow medium from the continuous evaporator heating surface;
wherein,
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at or immediately after the inlet to the steam generator tubes, the flow
medium is
subjected to strong heating such that it exhibits, in a first downcomer tube
section of
the relevant steam generator tube, a flow rate of more than a predefined
minimum
rate.
BRIEF DESCRIPTION OF THE DRAWINGS
An exemplary embodiment of the invention will now be explained in greater
detail
with reference to the accompanying drawing. Said FIG is a simplified view in
longitudinal section of a horizontally constructed continuous steam generator.
DETAILED DESCRIPTION OF THE INVENTION
The continuous steam generator 1 according to the FIG is connected downstream
of
a gas turbine (not shown) on the exhaust gas side in the manner of a waste-
heat
steam generator. The continuous steam generator 1 has a surrounding wall 2
which
forms a heating gas duct 6 for the exhaust gas from the gas turbine, heating
gas
flowing through said duct 6 in an approximately horizontal direction x
indicated by the
arrows 4. In the heating gas duct 6 there are disposed a number of heating
surfaces
designed according to the continuous principle, also termed continuous
evaporator
heating surface 8. Although only one continuous evaporator
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heating surface 8 is shown in the example depicted in the FIG,
a larger number of continuous evaporator heating surfaces can
also be provided.
The evaporator system formed by the continuous evaporator
heating surface 8 can be impinged by flow medium W which
evaporates in a single pass through the continuous evaporator
heating surface 8 and, on leaving the continuous evaporator
heating surface 8, is educted as steam D and generally fed to
superheater heating surfaces for further superheating. The
evaporator system formed by the continuous evaporator heating
surface 8 is connected into the water-steam circuit of a steam
turbine (not shown in greater detail). In addition to the
evaporator system, the water-steam circuit of the steam
turbine contains a number of other heating surfaces not shown
in greater detail in the FIG. The heating surfaces can be e.g.
superheaters, medium pressure evaporators, low-pressure
evaporators and/or economizers.
The continuous evaporator heating surface 8 of the continuous
steam generator 1 according to the FIG comprises, in the
manner of a tube bundle, a plurality of parallel-connected
steam generator tubes 12 providing a flow path for the flow
medium W. A plurality of steam generator tubes 12 is disposed
side by side viewed in the heating gas direction x, only one
of the thus disposed steam generator tubes 12 being visible.
On the flow medium side, the steam generator tubes 12 thus
disposed side by side are preceded upstream of their inlet 13
to the heating gas duct 6 by a common inlet header 14 and
followed downstream of their outlet 16 from the heating gas
duct 6 by a common outlet header 18. The steam generator tubes
12 comprise a plurality of riser tube sections 20 through
which flow medium W flows in the upward direction and
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downcomer tube sections 22 through which it flows in the
downward direction, these being interconnected by crossflow
sections 24 through which the flow medium W flows
horizontally.
The continuous steam generator 1 is designed for particularly
high operating reliability and consistent suppression of
significant temperature differences (also termed temperature
unbalance) at the outlet 16 between adjacent steam generator
tubes 12 even when the steam generator is fed with
comparatively high mass flow densities. For this purpose the
continuous evaporator heating surface 8 comprises, in its
downstream region viewed from the flow medium side, a heating
surface segment 26 connected countercurrently to the heating
gas direction x. A number of riser tube sections 20 and
downcomer tube sections 22 interconnected by crossflow
sections 24 additionally form a further heating surface
segment 28 connected cocurrently with the heating gas
direction x upstream of the heating surface element 26. This
configuration means that the positioning of the outlet 16 is
selectable in the heating gas direction x. This positioning
can be selected for the continuous steam generator 1 in such a
way that the pressure-dependent saturated steam temperature of
the flow medium W arising in the continuous evaporator heating
surface 8 during operation deviates by less than a specified
maximum deviation of approximately 50C from the heating gas
temperature obtaining at the position or at the height of the
outlet 16 of the heating surface segment 26 during operation.
As the temperature of the flow medium W at the outlet 16 must
always be at least equal to the saturated steam temperature,
but on the other hand may be higher than the heating gas
temperature obtaining at this point, the possible temperature
differences between differentially heated tubes can be limited
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to the specified maximum deviation of approximately 50'C
without additional countermeasures.
The heating surface segment 28 disposed well upstream in the
heating gas duct 6 in the heating gas direction x is therefore
followed on the heating gas and flow medium side by the
heating surface segment 26 likewise formed from a number of
riser tube sections 20 and downcomer tube sections 22
interconnected by crossflow sections 24 and through which the
flow medium flows countercurrently to the heating gas
direction x.
An arrangement of tube sections though which flow medium flows
in the downward direction, like the downcomer tube sections 22
inside the heating gas duct 6, is basically only possible if
the stability of the flow within the steam generator tubes 12
is ensured by suitable measures. Heating of tube sections
through which flow medium flows in the downward direction
tends to result in the formation of steam bubbles in the flow
medium W which, if they rise against the flow direction of the
flow medium W because of their low specific gravity, may
adversely affect flow stability and therefore the operational
reliability of the continuous steam generator 1. On the other
hand, a configuration of the steam generator tubes 12 whereby
only the tube sections through which flow medium flows in the
upward direction, i.e. the risers 20, are heated, involves
high construction costs.
A particularly simple and therefore also robust type of
construction of the continuous steam generator 1 can be
achieved by making the continuous evaporator heating surface 8
particularly simple in respect of collecting and distributing
the flow medium W and eliminating additional components such
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as collecting tubes. Instead of this, the steam generator
tubes 12 incorporate a plurality of alternating riser 20 and
downcomer tube sections 22 connected in series on the flow
medium side which are mounted inside the heating gas duct 6,
i.e. subjected to heating by the heating gas.
The inlet 13 is disposed at the gas-side inlet of the
continuous evaporator heating surface 8, i.e. in the heating
gas duct 6 well upstream in the heating gas direction x. By
disposing the inlet 13 in the area of the heating gas duct 6
in which the heating gas has the highest temperature, very
rapid heating and therefore evaporation of the flow medium W
in the steam generator tubes 12 is achieved. As the flow rate
of the water-steam mixture, the mass flow rate being equal, is
higher the greater the steam portion and therefore the
specific volume of the mixture, with this arrangement of the
inlet header 14 the flow medium W attains a high flow rate
comparatively quickly.
This is particularly favorable in order to ensure the
stability of the flow taking place in the steam generator
tubes 12. An important factor severely detrimental to flow
stability is the occurrence of steam bubbles in the steam
generator tubes 12. Because of their low specific weight, gas
bubbles forming may rise upward in the steam generator tubes
12, thereby moving against the flow direction in the downcomer
tube section 22. As a movement of this kind would seriously
impair flow stability, the rising of steam bubbles produced in
the steam generator tubes 12 must be consistently prevented.
An important criterion for flow stability is the flow rate of
the flow medium W. If, in the first tube section through which
the flow medium flows in the downward direction, i.e. in the
first downcomer 22, it already has a value at least as high as
